All the work done on making the adjusting has to be made off the ground, so mine is done while it's in my truck bed.
Tension adjustment is the job of the knob under the steering handle. The control lines attach to the knob's bolt and twist around it as you turn the knob. You should be able to do this easily while seated in the kayak... on the water. Figuring out which way to turn and how much is a process you have to just try. The lines will be loose when the knob is in its "centered" position. The lines will get tighter when the knob is turned in either direction.